Drug resistance is a significant factor in tumor chemotherapy and is also a useful marker for the genetic analysis of mammalian cells. Resistance to adenine analogs in somatic cells in vitro is most frequently associated with a lack of adenine phosphoribosyltransferase (APRT) activity. The genetic basis of this phenotypic deficiency in mammalian cells in uncertain. This project will initially assume that the APRT phenotype may arise from point mutations in the aprt gene or form stable epigenetic changes. To distinguish between these two possibilities a large number of APRT rodent cell clones will be isolated, characterized and fused to APRT human cells, some of which may carry nonsense suppressor genes and/or genes capable of activating (derepressing) the rodent aprt gene. Analysis of the APRT in those combinations that yield APRT ion hybrids at a high frequency will suggest the etiology of the APRT deficiency in the rodent parent. In particular, nonsense mutations in the aprt gene will be distinguished from mechanisms of phenotypic variation involving the loss of function of regulatory genes. Additionally, segregation analysis of the hybrids will permit the genetic mapping of human nonsense suppressor and/or regulatory genes.